5 Answers
5

Wikipedia says lenses below 24mm focal length (in 35mm-equivalent) are considered ultra-wide. Personally I'd say that the field of view becomes ultra-wide when people near the border of the picture start to look significantly wrong.

In Michael Freeman's The Photographer's Mind, he mentions that consumer-level ultra-wide angle lenses (which he defines as 24mm and below) for 35mm cameras started being widely produced in the 1960s. This fits with what I can see from researching lens history — the retrofocal lens was imported from cinema use (where it had been invented in the 1930s) in about 1950, and over the next decade became widely available from different manufacturers.

In 1958, the journal Photogrammetric Engineering has an article where, in the context of aerial photography, "ultra wide-angle" is defined as having angular coverage greater than 120°. That'd be something like 12.5mm on the 35mm film frame, or about 8mm on APS-C. But that's a technical application. As the marketers starting trying to sell wider-angle lenses to consumers, the numbers crept up considerably, and I found several instances of companies calling 28mm lenses ultra wide-angle — this Minolta book for beginners, for example.

I also found numerous other references to 24mm as being the cut-off point (like this excerpt from Time-Life's The Camera). So, while the Wikipedia article doesn't (currently) give any sources for its claim, it looks like there's pretty broad consensus.

It's interesting to note (though perhaps not directly relevant) that the accepted cutoff point coincides almost exactly with non-retrofocal view camera lenses that make movements (shift/rise, swing/tilt) impractical without using a "bag bellows" due to the physical limitations of ordinary bellows (there is some inherent stiffness when the bellows is collapsed). It's like the gods of perspective are trying to tell us something.
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user2719Mar 22 '11 at 18:33

Could you clarify this a bit, and perhaps back up your claims with some references or math?
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jrista♦Mar 21 '11 at 18:15

@jrista 16mm on an APS-C camera with a 1.5x crop factor (24mm on a 35mm equivalent system) is right in line with che's answer, which references Wikipedia. A quick check of the wiki page's references turns up a link to canon.com/camera-museum/camera/lens/ef/ultra_wide.html where a series of ultra wide lenses (EF mount) are listed, ranging from 14mm to 20mm.
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SeanMar 21 '11 at 21:29

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@Sean: I know that the answer is technically correct...but for the benefit of the community, I think it could stand to be fleshed out a bit more.
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jrista♦Mar 22 '11 at 5:20

A lens is considered a wide-angle lens when its focal length is shorter than the longer side of the sensor. A lens is considered an ultra-wide angle lens when its focal length is shorter than the shorter side of the sensor. (Wikipedia article) After all, it is the image sensor that dictates the Field-of-View over the focal length of a given lens.

This means that a lens can be "ultra-wide" when used with one camera and just a "wide-angle" when the same lens is attached to another camera, depending on the size of the sensor in each camera.

Four Thirds sensor size is about 17mm x 13mm and that means a lens with 14mm focal length is considered just a wide-angle lens, not ultra-wide, when used on a Four Thirds camera.

APS-C sensor size is roughly 24mm x 16mm, so any lens under 16mm focal length can be called ultra-wide angle lens on an APS-C format camera.

Full frame sensor size is 35mm x 24mm and that would mean a lens with 34mm focal length is considered wide-angle lens and anything shorter than 24mm is then ultra-wide angle for a full frame camera.

What is the point at which a lens is considered "ultra-wide angle" vs just wide angle?
It is at the length of the shorter side of the sensor in your camera.